Gas cleaning apparatus

ABSTRACT

Apparatus for separating particulate solid material from a stream of gas, such as air, entraining such material includes a blower, a solids collection receptacle, ducting connected to the blower and to the receptacle and communicating with a source of gas with entrained particulate solid material, the blower, receptacle and ducting providing a substantially closed system in which gas is recirculated, the solid material being separated from the circulating gas and deposited in the collection receptacle. A portion of the circulating gas, usually not exceeding 30 percent by volume, is bled from the circulatory system and discharged to maintain the average pressure in the circulating system at a desired low value so as to prevent leakage blow-out of gas and entrained dust or fine solids. Means for bleeding gas off from the system is situated within structure housing the blower and a separation chamber. The solids collecting receptacle is mounted so as to be bodily movable away from the blower and ducting housing structure and so as to respond to such bodily movement by tipping to dump accumulated solids. The gas cleaning apparatus is disclosed in connection with mobile surface sweeping equipment including a pick-up head having means for regulating the flow of air from a pressure chamber against the surface being cleaned and into a suction chamber in the pick-up head.

llite it tates Ftet 1 1 Willias Sept. 4,1973

[ GAS LlEANlNG APPARATUS [75] Inventor: John K. Williams, Waco, Text[73] Assignee: Central Texas Iron Works, Inc.,

Waco, Tex.

22 Filed: Oct. 28, 1971 211 Appl. No.: 193,514

[52] US. Cl 15/346, 15/340, 55/429 Primary Examiner-John PetrakesAssistant Examiner-C. K. Moore Att0rneyDonald M. Wight et a1.

[ ABSTRACT Apparatus for separating particulate solid material from astream of gas, such as air, entraining such material includes a blower,a solids collection receptacle, ducting connected to the blower and tothe receptacle and communicating with a source of gas with entrainedparticulate solid material, the blower, receptacle and ducting providinga substantially closed system in which gas is recirculated, the solidmaterial being separated from the circulating gas and deposited in thecollection receptacle. A portion of the circulating gas, usually notexceeding 30 percent by volume, is bled from the circulatory system anddischarged to maintain the average pressure in the circulating system ata desired low value so as to prevent leakage blow-out of gas andentrained dust or fine solids. Means for bleeding gas off from thesystem is situated within structure housing the blower and a separationchamber. The solids collecting receptacle is mounted so as to be bodilymovable away from the blower and ducting housing structure and so as torespond to such bodily movement by tipping to dump accumulated solids.The gas cleaning apparatus is disclosed in connection with mobilesurface sweeping equipment including a pick-up head having means forregulating the flow of air from a pressure chamber against the surfacebeing cleaned and into a suction chamber in the pick-up head.

33 Claims, 15 Drawing Figures Patented Sept. 4, 1973 5 Sheets-Sheet 1INVENTOR JOHN K. WILLIAMS BY m 1M1 ATTORNEY Patented Sept. 4, 1973 5Sheetg-Sheet 2 h l n 5m Patented Sept, 4, 1973 5 Sheets-Sheet 5 tentedSeit... ii

5 Sheets-Sheet 4 Patented Sept. 4, W73

5 Shwts-Sheet 5 HUM GAS CLEANING APPARATUS This invention relates toapparatus for separating gas from particulate solid matter entrained ina stream of the gas. More particularly the invention providesimprovements in previously known separation apparatus in whichparticulate solid matter is picked up in a stream of gas, e.g., air, andconducted in a substantially closed recirculating air stream system toseparating means including a receptacle into which solid matterseparated from the air stream is deposited. Such recirculating systemsare used, for example, in connection with street or other surfacecleaning equipments in which the cleaning action is performed mainly ifnot entirely by air stream action, as distinguished from by brushing thesurface mechanically. While such recirculating systems are intended tobe, and in practice essentially are, closed, it is inevitable that some,although relatively very little, air is drawn from outside the systeminto the suction side of the system which thereby increases the totalamount of air in the system. Since the system is essentially of fixedvolume, the drawing in of additional air tends to increase the averagepressure throughout the system. This increase in pressure will causesome air with entrained dust to be discharged from the high pressureside of the circulation system through leaks in the ducting orcollection receptacle or from the pressure chamber of the pick-up head.Air leaking under pressure carries with it entrained dust or otherparticles, discharging of which to the atmosphere is naturallyundesirable, in fact in many situations, intolerable.

An object of the present invention is to provide an apparatus of thegeneral character outlined above but which includes a new and improvedarrangement of ducting and valve means by which a relatively smallportion of the gas circulating in the otherwise closed system is bledoff and discharged from the system, thereby avoiding the building up ofthe average pressure throughout the system and preventing dust or othersolid matter from being blown out of the system.

Another object of the invention is to provide such an apparatusincluding a filter through which gas bled off from the system isdischarged, and in which the bleedoff valve means is operable forreversing flow of gas through the filter to dislodge solid matter fromthe filter and deliver it to the suction side of the recirculationsystem.

Another object of the invention is to provide a recirculation gascleaning apparatus including an improved separation chamber fitted witha curved baffle defining a skimmer slit for removing solids from awhirling stream of gas.

Another object of the invention is to provide a surface cleaning pick-uphead for use in connection with a recirculating system as describedabove, the pick-up head being provided with means for regulating theblasting of air within the pick-up head from a pressure chamber to asuction chamber.

A further object of the invention is to provide means for mounting thesolids collection receptacle of a system as outlined above in suchmanner that the receptacle may be moved bodily away from a blower andducting housing and, as an incident to such bodily movement, will betipped to discharge collected solids.

Other objects of the invention will become apparent from a reading ofthe following description of a preferred embodiment, the appendedclaims, and the accompanying drawings, in which:

FIG. I is a somewhat schematic side elevational view of mobile surfacesweeping equipment comprising a truck and apparatus for separatingentrained solids from air supported thereon;

FIG. 2 is a top plan view of the equipment shown in FIG. 1;

FIG. 3 is a longitudinal vertical section on the line 3-3 of FIG. 2;

FIG. 4 is a vertical section on the line 4-4 of FIG.

FIG. 5 is a vertical section on the line 55 of FIG. 2;

FIG. 6 is a horizontal section on the line 6--6 of FIG.

FIG. 7 is an enlarged scale horizontal section on the line 77 of FIG. 4,an air bleed-off valve being shown in position for bleeding air from a.main air circulatory system;

FIG. 8 is a view similar to FIG. 7, but showing the valve in a differentposition, for reverse flow cleaning of a filter;

FIG. 9 is a perspective view, with parts broken away, showing the insideof a housing with a blower and ducts and separator chamber beingillustrated;

FIG. 10 is an enlarged scale cross section of a pick-up head;

FIG. 11 is a fragmentary longitudinal section of the pick-up head on theline ll1l in FIG. 10, with parts broken away, and drawn on a smallerscale than FIG. 10;

FIG. 12 is a perspective view of an angle plate member included in thepick-up head construction; and

FIGS. 13A, 13B and 13C are diagrammatic elevational views showingsuccessive positions of a solids collection receptacle during a dumpingoperation.

A representative embodiment of the invention is shown as being mountedon a mobile truck or the like permitting the equipment to be moved overa street or other surface to be cleaned. In general, surface cleaning iseffected by directing a blast of air against the surface to entrainparticulate solid matter in an airstrearn which is then delivered toequipment for separating the solid particulate matter from the airstreamand for then returning the air under pressure so as to recirculatecontinuously in the cleaning of the surface. The equipment is shown asbeing mounted on a wheeled truck T having a frame 1 for supporting theequipment for picking up solid particlesin the airstream and separatingthe particles from the air. An operators seat 2 and steering wheel 3 areshown conventionally. The truck T may be driven by any conventionalmeans.

Considering the surface cleaning and air and solids separating equipmentgenerally, a blower B receives its intake of air from a solidscollection receptacle SCR, thereby reducing the pressure in thereceptacle to a relatively negative pressure. The blower delivers airunder pressure through a duct 4to a pressure chamber PC in a hollowpick-up head structure PI-I, the construction of which is described moreparticularly hereinafter. An air blast is delivered from the pressurechamber PC downwardly against the surface to be cleaned and thence to asuction chamber SC in the pick-up head. Air with entrained particulatematter flows from the suction chamber SC to the solids collectionreceptacle SCR through a suction duct 5 and further ducting means to bedescribed later. The air with entrained solid matter entering thecollection chamber SCR is reversed in its flow in the receptacle so thatsome of the solid matter drops out of the airstream into the bottom ofthe receptacle. Air thus at least partially cleaned flows to the blowerintake via supplemental separating structure generally designated 6 inFIGS. 3, 6 and 9 and which will be further described hereinafter. Airflows from the supplemental separating structure 6 to the blower intake,thus completing the circulation which is repeated continuously over thesame system route.

Considering the illustrative construction more particularly, a housinggenerally designated 7 encloses the blower B and provides ductingrequired for the air to flow over the recirculating path generallydescribed above. The housing 7, which is mounted on the support 1,includes side walls 8 and 9, a front wall 10, a bottom wall '11, and atop wall 12. The rear face of the housing 7 is open, that is unwalled.

Mounted in the housing 7 are two vertical partitions 13 and 14 extendingfore-and-aft between the front wall and the open rear face of thehousing. Rotatably mounted in the space between the partitions l3 and 14is the blower impeller 15 driven through a shaft 16 by an engine 17 andclutch 18 generally indicated in FIG. 2. As shown in FIGS. 4 and 9, acurved shroud 19 is mounted between the partitions 13 and 14 so aspartially to surround the impeller 15. Also, as shown in FIG. 4, aretransition members 20 and 21 mounted between the partitions 13 and 14and extending downwardly from the shroud 19 to the pressure duct 4leading to the pressure chamber PC of the pick-up head PH. Airdischarged peripherally by the impeller 15 is thus delivered downwardlythrough the pressure duct 4 to the pressure chamber PC.

The space 22 in the housing 7 between the housing side wall 9 and thepartition 14 serves as part of duct means communicating with the suctionduct 5 and suction chamber SC of the pick-up head PH, the space 22 beingopen at the rear face of the housing 7.

As shown in FIGS. 2, 3, 6 and 9, the space between the partition 13 andthe housing side wall 8 serves as a separation chamber 23 which isuseful for removing from air flowing to the blower any entrained solidmatter which was not deposited in the collection receptacle SCR.

The chamber 23 is open at its rear to the receptacle SCR andcommunicates with the blower intake through an inlet opening 24 in thepartition 13. Mounted in the separation chamber 23 is a curved partition25 with a rearward-downward extension 26 projecting to the rear of thehousing 7 as clearly shown in FIG. 3. The curved partition 25 followsgenerally the curvature of a curved part 10a of the housing front wall10.

Mounted in the separation chamber 23 is a curved baffle 27 generallyfollowing the curvature of the partition 25. The upper edge 28 of thebaffle 27 is spaced substantially below the top 12 of the housing 7,whereas the lower or opposite edge 29 of the baffle 27 is spaced frombut is relatively very close to the mergence of the curved partition 25with its extension 26. The relatively large spacing of the baffle edge28 from the top 12 of the housing provides for free flow or air from thecollection receptacle SCR tangentially into the separation chamber 23 soas to create a swirling or vortex action which centrifugallyconcentrates any remaining solid particles out against the wall 10a, 25of the separation chamber 23. The concentrated solids are skimmed outthrough a skimmer slit 30 defined by the lower edge 29 of the curvedbaffle 27 and the adjacent part of the partition 25.

The solids collection receptacle SCR is mounted on the support 1 bymeans permitting it to be raised and dumped as later to be described. Atthis point it is sufficient to explain the construction of thecollection receptacle SCR as it is related to its functioning as part ofthe circulating system for separating solids from air. The receptacleincludes a top wall 31, side walls 32 and 33, a rear wall 34, and anobtusely angled bottom wall 35. The front face of the receptacle isopen, that is unwalled. When the receptacle is in operative position,its open front face permits free communication between the receptacleand the duct space 22 in the housing 7 between the side wall 9 and thepartition 14 of the housing. The open front face of the receptacle alsoplaces the receptacle into communication with the separator space 23included between the housing partition 13 and housing side wall 8 asdescribed above. As shown in FIG. 6, the housing 7 is provided with alaterally offset wall extension 36 which closes the open front face ofthe collection receptacle SCR beyond the housing wall 9.

As shown in FIGS. 3, 6 and 9, the collection receptacle SCR is fittedwith a screen partition 37 extending fore-and-aft and generally inalignment with the housing partition 13 and dividing the receptacle intotwo sections. Air with entrained solid matter enters the receptacle SCRfrom the duct space 22 and flows through the screen 37 before beingreturned to the blower intake 24 by way of the supplemental separationchamber 23. The screen 37 is useful for intercepting a considerableportion of the entrained solid matter before the circulating air isreturned to the blower.

One form of pick-up head suitable for use in connection with an aircleaning apparatus according to the invention is shown in FIGS. 10 and11, although other pick-up head constructions may be used. The pick-uphead PH as shown in FIGS. 10 and 11 includes, a stepped top plate wall38 provided with longitudinal flanges 39 and 40, and end panels 41. Ahorizontal wall 42 is welded or otherwise secured to the inside of theflange 39 spaced below the top wall 38 and extends toward thelongitudinal center of the pick-up head PH. The horizontal wall 42mounts two flexible or yieldable U-shaped members 43 formed with spaceddownwardly extending aprons or curtains 44. The members 43 are securedto the wall 42 by clamping strips 45 and bolts 46.

Underneath the opposite edge of the pick-up head PH, that is adjacent tothe flange 40, is mounted another flexible member 47 generally U-shapedin cross section and comprising aprons or curtains 48. A channel-shapedstrip 49 and bolts 50 clamp the member 47 to the bottom face of the topwall 38.

The end panels 41 are provided with skids 41a which rest upon thesurface being cleaned and support the pick-up head PH. The curtains 44and 48 and the end panels 41 together enclose a working air spaceincluding both the pressure chamber PC and the suction chamber SC. Thecurtains assist in supporting the pickup head, but the curtains 48 areflexible enough to yield upwardly and admit cans and other objects intothe pick-up head working space. As shown in FIGS. 1 and 6, the pick-uphead PH is towed along in the direction of cleaning movement progress bydraw bars 51 connected at their rear ends to the pick-up head PHandconnected at their front ends to the truck frame ll.

Extending generally centrally and longitudinally inside the pick-up headand transversely to the direction of cleaning movement progress is aninclined parition generally designated 52 which separates the suctionchamber SC from the pressure chamber PC. The partition comprises a plate53 secured to the inner face of the step-down part 38a of the top platewall 38, and an inclined flange 42a extending along the front edge ofthe horizontal plate 42. The flange 42a is parallel to and spaced fromthe lower portion of the plate 53 so as to provide an elongated orificeslit 54 for discharging blast air from the pressure chamber PC againstthe surface being cleaned at a downward-forward inclination and thenceinto the suction chamber SC. The spacing of the lower part of the plate53 from the flange 42a and hence the width of the slit 54 are fixed bytwo spacers 55 at the respective ends of the orifice slit between theplate 53 and the flange 42a, and two intermediate spacers 56 between theplate 53 and the flange 42a. The end spacers 55 have downwardly inwardlysloping edges as shown in FIG. 11. These edges direct blast air emergingfrom the slit orifice inwardly and away from the respectively adjacentends of the suction chamber toward the longitudinal center of thelatter.

At each end of the suction chamber SC is an angle plate member '57secured to the inside of the adjacent end panel 41. One angle platemember 57 is shown in FIG. 12 as having its vertical web 57a secured tothe inner face of the one end panel 411 and having its horizontal web57b extending inwardly from that panel into the suction chamber. Theother angle plate member 57 (not shown) is similarly mounted on theother end panel 411. The horizontal webs 57b of the members 57 arechamfered 45 at 57c as shown also in FIG. 12. The plate members 57divert blast air from ends of the suction chamber toward its center andthus prevent blowout of blast air at the ends of the suction chamber.

In operation, air delivered under pressure by the blower B flowsdownwardly through the pressure duct 4 into the pressure chamber PC ofthe pick-up head. Air under pressure in the pressure chamber flows athigh velocity in the manner of a blast through the slit orifice 54 toimpinge upon the surface being cleaned at an angle so as to dislodgeparticles from the surface and disperse them in the suction chamber SC.Air with entrained particulate solid material is then drawn from thesuction chamber up through the suction duct 5 into the duct space 22 inthe housing 7 and thence into the collection receptacle SCR between thereceptacle side wall 33 and the screen 37, a considerable portion of theentrained solid material descending to the bottom of that part of thereceptacle by gravity. The screen 37 intercepts non-descended solidparticles from the airstream as it passes into the part of thecollection receptacle between the screen 37 and the side wall 32 of thereceptacle. The airstream flows from the latter part of the receptacleinto the supplemental separation chamber 23 and thence through theblower intake 24, thus completing the recirculation flow path. Very finesolid particles remaining in the airstream flowing from the receptacleSCR into the supplemental separation chamber 23 are largely removed bythe centrifugal action resulting in skimming off of fine solid particlesthrough the skimmer slit 30 and returning of those particles downwardlyalong the partition extension 26 to the collection receptacle.

Although the recirculation system described above is essentially aclosed system, as a practical matter some air in addition to that beingcirculated inevitably is drawn into the system where under suctionconditions, principally at the ground/curtain interface. Air may also bedrawn in in lesser degree through any leaks in imperfectly sealed jointsin the housing 7 defining the suction duct space 22, andwhere thecollection receptacle SCR fits against the housing 7 without perfectsealing. Whatever additional air is drawn into the closedsystem tends toraise the average system pressureabove atmospheric pressure. This meansthat there must be some discharge of air from the system to compensatefor that drawn in. Emitting air through leakage or say under thecurtains 44 adjoining the pressure chamber PC would result in blowingdirty air into the atmosphere which would obviously be undesirable andwhich would result in sacrificing advantages otherwise obtainable by theuse of the closed circulating system.

The construction illustrated and described above has provision fordischarging or bleeding off air in an amount approximately equal to thatdrawn in incidentally so as to maintain the average system pressure at adesired low level. The air discharged or bledoff is delivered to theatmosphere without discharging significant amounts of dust or otherparticles freely to the at mosphere. This is accomplished by by-passingor bleeding some of the circulating air, which may contain someentrained solid particles, through a filter by means of equipment now tobe described.

A by-pass or pressure relief plenum structure generally designated 64 ismounted on the housing 7 adjacent the top of the curved part 10a of thehousing front wall 10. The plenum structure 64 includes a top wall 65, abottom walli66, a curved end wall 67, an outer side wall 68, and aninner side wall 69. The plenum bottom wall 66 abuts the curved part 10aof the housing front wall 10 as shown in FIGS. 4, 5 and 9. The sidewalls 68 and 69 conform at their rear ends to the curvature of and theyabut the curved wall part 10a of the housing front wall. The plenumstructure 64 includes an additional wall 70 which is spaced inwardlyfrom the wall 69 as shown in FIGS. 7 and 8 so as to provide a flowpassage 71 leading tangentially into the round part of the plenum.

A pressure relief or by-pass opening 72 is formed in the blower casingadjacent the juncture of the housing front wall 10 and the top wall 12as shown in FIGS. 2, 4 and 9. The pressure relief opening 72communicates with the flow passage 71 of the plenum 64.

A suction port 73 is formed in the housing 7 adjacent the convergence ofthe front wall 10a with the housing top wall 12 so as to communicatewith the ducting space 22 previously described as being between thehousing wall 9 and the partition 14. The port 73 is also adapted tocommunicate with the plenum 64 at a zone spaced circumferentially fromthe tangential reception of air entering the plenum from the passage 71.

At the center of the plenum 64 is a gas bleed outlet extending through aspigot 74 on which is mounted the open end part of a filter bag 75 offoraminous material.

A segment-shaped by-pass control valve 76 is pivoted at 77 at the centerof curvature of the plenum wall 67 and is operable by a handle 79located above the plenum 64 as shown in FIGS. 2 and 3 so as to bemovable to two positions as shown respectively in FIGS. 7 and 8.

Normally, that is during most periods of operation, the valve 76 will bepositioned as shown in FIG. 7 so that there is free opening for air withentrained dust to move from the relief opening 72 and passage 71 intothe plenum circular part tangentially. In this position of the valve 76,an edge 76a of the valve is closely adjacent to the plenum wall 68 so asto prevent free communication between the plenum and the port 73, butleaves a narrow skimmer slit between the valve edge 76a and the plenumwall 68. The slit provides for controlled restricted communication ofthe port 73 with the plenum 64 at a zone spaced circumferentiallyapproximately 180 from the tangential reception of bypassed air by theplenum. Some of the dirt laden air flowing circularly in the plenum willbe relieved of its dust by centrifugal force concentrating the dustparticles in the outer part of the plenum from which they are passedthrough the skimmer slit at the valve edge 76a with a small amount ofair. The air and dust passing through the skimmer slit are returned tothe system through the port 73, suction duct space 22, and collectionreceptacle SCR. However, the major part of the air received in theplenum 64 from the relief opening 72 flows to the plenum center and outthrough the bleed outlet 74 into the filter bag 75. The bag traps andretains practically all of the entrained dust while the bledoff airpasses out through the bag to the atomsphere without harmful andunpleasant results.

After operating for some time with the valve 76 in the position shown inFIG. 7 with bleeding off of surplus air as described above, the filterbag 75 normally will accumulate a considerable amount of dust or otherparticles which, if not removed, would reduce the efficiency of thefilter. Accordingly, it is desirable to provide for cleaning the filter.Of course, this could be done by removing the bag 75 and replacing itwith a clean bag, but this would involve additional trouble and expenseand extra equipment. The present construction provides for cleaning thefilter bag 75 by the simple expedient of moving the valve 76 to theposition shown in FIG. 8 in which the valve edge part 76b opposite theedge 76a blocks off the passage 71 and the valve edge part 76a is widelyspaced from the plenum wall 68 so as to place the curved part of theplenum and also the interior of the filter bag 75 in substantiallyunrestructed communication with the port 73 and hence with the suctionduct space 22 and receptacle SCR. The inside of the bag 75 is therebysubjected to suction which causes the bag to give up dust or otherparticles residing in the bag. Removal of the particles from the bag isenhanced by flexing of the bag resulting from reversal of flow of airinto the bag from the atmosphere, as distinguished from the normal flowoutwardly through the bag into the atmosphere. The air drawn into thebag 75 from the atmosphere entrains the dust dislodged from the bagwalls and carries it by suction flow through the port 73 and duct space22 into the receptacle SCR.

The setting of the valve 76 in the FIG. 8 position for cleaning thefilter bag is used only periodically and for relatively short periods ascompared to the operation under the normal conditions when the valve ispositioned as shown in FIG. 7. The frequency of reverse cleaning thefilter by positioning the valve as shown in FIG. 8 may be reduced byconnecting the bottom of the filter bag to the suction side of therecirculatory system.

In the form shown, the bottom of the bag is connected to a tube 75acommunicating with the suction chamber SC in the pick-up head PH. Dustor solids in the filter bag which do not adhere to the bag side wallsbut drop to the bottom of the bag are thus returned directly to thesuction side of the system while the valve 76 is in its normal positionas shown in FIG. 7. Suction removal of solid particles from the bottomof the bag 75 is improved by provision of a disk cover 75b on the tube75a formed with a restricted air passage hole 750. Air drawn by suctionfrom the bag 75 into the suction chamber SC must flow at a high velocitythrough the hole 75c so as efficiently to entrain particles and carrythem into the tube 75a and thence to the suction chamber SC.

The solids collection receptacle SCR is mounted so as to be supported onthe truck frame 1 and to be moved from its normal position shown in FIG.1 bodily away from the housing 7 and simultaneously to be tipped so asto discharge material collected in the receptacle to the rear andthrough the open face 78 of the receptacle. In the normal position ofthe receptacle, its open front face 78 communicates directly with thesuction duct space 22 and with the separation chamber 23 in the housing7. When the receptacle is moved bodily away from the housing 7 and istipped, the housing contents are dumped or discharged through the openface 78 of the receptacle, thus dispensing with the necessity ofproviding a separate clean-out door or gate.

As shown in FIG. 1, control means for tipping the receptacle when it ismoved away from the housing includes a relatively long link 79 pivotedat 80 to a bracket 81 on the support 1, and pivoted at 82 to thereceptacle SCR relatively close to the receptacle open face 78 and nearthe top of the receptacle.

A relatively short link 83 extending above the link 79 is pivoted at 86to the receptacle side wall 32 close to the receptacle top wall 31. Thelinks 79 and 83 together with the support 1 and the receptacle SCRconstitute a four-bar linkage.

Means for operating the control linkage includes a power operableextensible assembly generally designated 87 and in the present instanceillustrated as an assembly of a cylinder 88 and piston rod 89 operableunder fluid pressure from a source, not shown. The cylinder 88 ispivoted at 90 on the support 1, and the piston rod is pivoted at 91 toan intermediate part of the link 79. Fluid pressure extension of thepiston rod 89 operates through the links 79 and 83 and other componentsof the four-bar linkage to move the receptacle SCR from the normalposition shown in FIG. 1 successively through the positions shown inFIGS. 13A, 13B and 13C, thus moving the receptacle SCR bodily away fromthe housing 7. The control linkage responds to movement of thereceptacle away from the housing by tipping the receptacle so far as todump the contents through the same open face 78 as was positionedagainst the open face of the housing 7 during normal operation. Thelinkage and operating means located adjacent the side 32 of thecollection receptacle SCR have been described. For balancing forces, asimilar mechanism is provided at the side 33 of the receptacle.

When the receptacle SCR is in its normal position with its open frontface adjacent the generally vertically extending open rear face of thehousing 7, flanges 92 surrounding the front face of the receptacle abutsealing gaskets 93 which surround the rear face of the housing 7. Thisprovides a sufficiently good seal to pre' vent inward leakage of moreair than can be compen sated for by the bleeding 011 of air through thefilter 75. The screen 37 is secured to the receptacle SCR walls 31, 34and 35 by strips 94. As shown in FIGS. 3 and 6, two flanged strips 95extend along the front edge of the screen 37. When the receptacle is inits normal position as shown in FIGS. 1, 3 and 6, the flanges of thestrips 95 fit over and embrace a rearward extension 13a of the housingpartition 13. in this manner, the front edge of the screen 37 issupported against lateral deflection, for example in the event of pilingof dust and even heavier particles against one side of the screen.

Although the invention has been described in its application to airblast surface cleaning, it will be understood that apparatus accordingto the invention may be used for separating particulate solid materialfrom air or other gas generally, and not only in connection with streetor other surface cleaning operations. The construction illustrated anddescribed embodies the invention in a preferred form, but it is intendedthat the disclosure be illustrative rather than definitive, theinvention being defined in the claims.

I claim:

1. ln apparatus for separating gas from particulate solid matterentrained therein and having a solids collection receptacle; a blower;means providing communication between the intake of said blower and saidsolids collection receptacle whereby to place the interior of saidsolids collection receptacle underrelatively negative pressure; a hollowstructure for receiving particulate solid matter for entrainment in agas flow stream;

first duct means affording communication of the:

blower discharge with said hollow structure; and second ductmeansiafiording communication of said solids collection receptacle withsaid hollow structure; said solids collection receptacle, said blower,said hollow structure and said first and second duct. means. providing asystem for recirculation of gas in a flow path from said hollowstructure through said second duct means, said solids collectionreceptacle, said blower and said first duct means back to said hollowstructure and continuously over said path, the intake of said blower,said solids collection receptacle, said second duct means and a part ofsaid hollow structure being under'saidrelatively negative pressure,whereby some gas inaddition to the recirculated gas enters into saidflow path from outside said system; the improvement comprising gasbleeding means for preventing said addition of gas from outside saidsystem from undesirably increasing the average gas pressure in saidsystem, said gas'bleeding means including a pressure relief plenum; aby-pass opening in said first duct means communicating with saidpressure relief plenum; a gas bleed outlet in said. plenum; and a valvein said plenumifor variably controlling the flow of gas therethroughfrom said by-pass opening to and through said gas bleed outlet.

2. Apparatus according to claim 1 including a filter mounted to receivegas discharged through said gas bleed outlet.

3. Apparatus according to claim 2 including a port in said second ductmeans communicating with said. plenum, said valve being operable forselectively placing said plenum respectively into communication withsaid by-pass opening in said first duct means and. with said port insaid second duct means, communication of said plenum with said port insaid second duct means 'providing for reverse flow of gas through saidfilter to dislodge solid material therefrom and carry it entrainedthrough said second duct means to said solids collec-' tion receptacle.

4. Apparatus according to claim 3 in which said filter comprises aforaminous bag into which gas is discharged through said gas bleedoutlet.

5. Apparatus according to claim. 1 in which said first duct meansincludes the casing of said blower, said bypass opening being formed insaid casing itself.

6. Apparatus according to claim 1 in which said plenum includes an outercylindrical wall portion located to receive by-passed gas tangentiallyfrom said by-pass opening to establish a swirling movement of gas andentrained particulate solid matter in said plenum, said gas bleed outletbeing at the central portion of said plenum; said apparatus furtherincluding a port in saidsecond duct means communicating with said plenumat a zone spaced circumferentially from the tangential reception ofby-passed gas by said plenum, said valve being selectively movable totwo positions in one of which positions it establishes relatively freeflow of gas from said by-pass openinginto said plenum and substantiallyprevents free flow of by-pass gas from said plenum to said port, and inthe other of which positions said valve substantially prevents flow ofgas from said by-pass opening into said plenum and establishesrelatively free flow of gas from said plenum to said port and thence tosaid second duct means and said solids collection receptacle.

7. Apparatus according to claim 6 including a foraminous bag filtermounted to receive gas discharged from said gas bleed outlet.

8. Apparatus according to claim 7 including means providingcommunication between said second duct means andthe interior of said bagfilter at a location spaced from said gas bleed outlet.

9. Apparatus according to claim 6 in which said valve comprises asegmental part substantially conforming to the shape of said outercylindrical wall portion, and means mounting said valve to turn aboutthe axis of said cylindrical wall portion and said segmental part.

10. Apparatus according to claim 9 in which, when said valve is in saidone of said two positions, a portion of said segmental part mainly cutsoff communication between said plenum and said port but an edge of saidsegmental portion is spaced slightly from said plenum wall to provide askimmer slit through which particulate solids centrifugally concentratedadjacent the inner surface of said plenum wall by the swirling of thegas within the plenum are discharged together with a minor portion ofthe by-passed gas to said port and thence to said second duct means andsaid solids collection receptacle, the major portion of the by-passedgas being discharged through said. gas bleed outlet.

11. Apparatus according to claim 10 including a hollow filter forreceiving by-passed gas discharged through said gas bleed outlet; andsolids return means for conducting a portion only of the gas andsolidsdelivered to said filter to said second duct means forreturn tosaid solids collection receptacle.

12. Apparatus according to claim 11 in which said filter extendsdownwardly from said gas bleed outlet, and in which said solids reutrnmeans is connected to the lower portion of said filter.

13. Apparatus according to claim 12 in which said solids return meanscommunicates with said second duct means via said hollow structure.

14. Apparatus according to claim 1 including a vertical screen partitionin said solids collection receptacle dividing the latter into twosections one of which communicates with the blower intake and the otherof which communicates with said second duct means.

15. Apparatus according to claim 1 including a separation chamber havinga curved wall in open communication with said collection receptacle; ablower inlet passage extending through a wall of said chamber to theblower intake; and a curved baffle in said chamber having one edgespaced relatively far from the adjacent chamber wal to providerelatively free flow of gas from said collection receptacle into saidchamber for being swirled therein as it moves inwardly to said inletpassage, the opposite edge of said curved baffle being spaced from butrelatively close to said chamber curved wall to provide a skimmer slitthrough which solid matter centrifugally concentrated in the outerportion of said chamber is returned to said collection receptacle.

16. Apparatus according to claim 15 in which said blower comprises animpeller rotatable about a horizontal axis, and in which said one edgeof said curved baffle is spaced below the top of said separation chamherto provide the relatively free flow of gas from the upper portion ofsaid collection receptacle into the upper portion of said separationchamber, and further in which said opposite edge of said curved baffleis at the lower portion of the separation chamber, whereby said skimmerslit is so located as to discharge into the lower portion of saidcollection receptacle.

17. In an air flow surface cleaning apparatus, a pickup head mounted formovement in the direction of cleaning progress over and adjacent to thesurface to be cleaned, said pick-up head including a pressure chamber; asuction chamber; an inclined partition between said chambers extendingtransversely of the direction of cleaning progress movement and havingan inclined slit orifice for the discharge of high velocity blast airfrom said pressure chamber against said surface and into said suctionchamber; and means at the ends of said slit orifice in the path of thehigh velocity blast air for directing blast air emerging from said slitorifice inwardly and away from the respectively adjacent ends of saidsuction chamber and toward the longitudinal cen- I ter of said suctionchamber.

18. Apparatus according to claim 17 in which said means at the ends ofsaid slit orifice comprise spacers within said slit orifice.

19. Apparatus according to claim 17 in which said means at the ends ofsaid slit orifice comprise members within said suction chamber at theopposite ends thereof and having horizontal webs with chamfered portionsin the path of the blast air emerging from the ends of the slit orifice.

20. Apparatus according to claim 17 in which said pick-up head has aunitary top wall comprising a relatively high pressure chamber top wallpart; a relatively low suction chamber top wall part; and an interveningstep-down part.

21. Apparatus according to claim 20 in which said intervening step-downpart is inclined at the same inclination as said inclined partition.

22. Apparatus according to claim 17 in which said pick-up head includesa horizontal wall forming the bottom of the pressure chamber and spacedabove the level of the surface being cleaned, said horizontal wallhaving an inclined edge flange providing one side of said slit orifice.

23. in an air flow surface cleaning equipment, a mobile support; apick-up head carried by said support and including a pressure chamberand a suction chamber; and apparatus for separating air from particulatesolid matter picked up and entrained by air flowing in said pick-up headfrom said pressure chamber to said suction chamber, said apparatus beingmounted on said mobile support and comprising a solids collectionreceptacle; a blower; means providing communication between the intakeof said blower and said receptacle; first duct means affordingcommunication of the blower discharge with said pick-up head pressurechamber; second duct means affording communication of said receptaclewith said pick-up head suction chamber, said receptacle, said blower,said pressure chamber, said suction chamber and said first and secondduct means providing a system for continuous recirculation of air in aflow path from said suction chamber through said second duct means, saidreceptacle, said blower and said first duct means to said pressurechamber and thence back to said suction chamber, the intake of saidblower, said solids collection receptacle, said second duct means andsaid pick-up head suction chamber being under relatively negativepressure, said recirculation flow being accompanied by some air enteringinto said flow path from outside said system and being added to therecirculated air flowing in said flow path; a pressure relief plenum; aby-pass opening in said first duct means communicating with saidpressure relief plenum; an air bleed outlet in said plenum; and a valvein said plenum for variably controlling the flow of air therethroughfrom said by-pass opening to and through said air bleed outlet.

24. Apparatus according to claim 23 including a filter mounted toreceive air discharged thnough said air bleed outlet.

25. Apparatus according to claim 24 including a port in said second ductmeans communicating with said plenum, said valve being operable forselectively placing said plenum respectively into communication withsaid by-pass opening in said first duct means and with said port in saidsecond duct means, communication of said plenum with said port in saidsecond duct means providing for reverse flow of air through said filterto dislodge solid material therefrom and carry it entrained through saidsecond duct means to said solids collection receptacle.

26. Apparatus according to claim 23 in which said first duct meansincludes the casing of said blower, said by-pass opening being formed insaid casing itself.

27. Apparatus according to claim 23 in which said plenum includes anouter cylindrical wall portion located to receive by-passed airtangentially from said bypass opening to establish a swirling movementof air and entrained particulate solid matter in said plenum, said airbleed outlet being at the central portion of said plenum; said apparatusfurther including a port in said second duct means communicating withsaid plenum at a zone spaced circumferentially from the tangentialreception of by-passed air by said plenum, said valve being selectivelymovable to two positions in one of which positions it establishesrelatively free flow of air from said by-pass opening into said plenumand substantially prevents free flow of bypass air from said plenum tosaid port, and in the other of which positions said valve substantiallyprevents flow of air from said by-pass opening into said plenum andestablishes relatively free flow of air from said plenum to said portand thence to said second duct means and said solids collectionreceptacle.

28. Apparatus according to claim 27 in which said valve comprises asegmental part substantially conforming to the shape of said outercylindrical wall portion, and means mounting said valve to turn aboutthe axis of said cylindrical wall portion and said segmental part.

29. Apparatus according to claim 8 in which, when said valve is in saidone of said two positions, a portion of said segmental part mainly cutsoff communication between said plenum and said port but an edge of saidsegmental portion is spaced slightly from said plenum wall to provide askimmer slit through which particulate solids centrifugally concentratedadjacent the inner surface of said plenum wall by the swirling of theair within the plenum are discharged together with a minor portion ofthe by-passed air to said port and thence to said second duct means andsaid solids collection receptacle, the major portion of the by-passedair being discharged through said air bleed outlet.

30. Apparatus according to claim 29 including a hollow filter forreceiving by-passed air discharged through said air bleed outlet; andsolids return means for conducting a portion only of the air and solidsdelivered to said filter to said second duct means for return to saidsolids collection receptacle.

31. Apparatus according to claim 30 in which said filter extendsdownwardly from said air bleed outlet, and in which said solids returnmeans is connected to the lower portion of said filter.

32. Apparatus according to claim 31 in which said solids return meanscommunicates with said second duct means via said pick-up head suctionchamber.

33. In apparatus for separating gas from particulate solid matterentrained therein and having a solids collection receptacle; a blower;means providing communication between the intake of said blower and saidsolids collection receptacle whereby to place the interior of saidsolids collection receptacle under relatively negative pressure; ahollow structure for receiving particulate solid matter for entrainmentin a gas flow stream; first duct means affording communication of theblower discharge with said hollow structure; and second duct meansaffording communication of said solids collection receptacle with saidhollow structure; said solids collection receptacle, said blower, saidhollow structure and said first and second duct means providing a systemfor recirculation of gas in a flow path from said hollow structurethrough said second duct means, said solids collection receptacle, saidblower and said first duct means back to said hollow structure andcontinuously over said path, the intake of said blower, said solidscollection receptacle, said second duct means and a part of said hollowstructure being under said relatively negative pressure, whereby somegas in addition to the recirculated gas enters into said flow path fromoutside said system; the improvement comprising gas bleeding means forpreventing saidl addition of gas from outside said system fromundesirably increasing the av erage gas pressure in said system, saidgas bleeding means including a by-pass opening in said first duct means;a gas bleed outlet communicating with said bypass opening; a filtermounted vertically to receive therein gas and any entrained particlesdischarged through said gas bleed outlet; and solids return means forconducting a portion of the gas: and solids delivered to said filterfrom the bottom of said filter to said second duct means for return tosaid solids collection receptacle, said solids return means comprising arestricted gas passage for causing said portion of gas and solids toflow at a high velocity so as to entrain particles collecting at thebottom of said filter and conduct them to said second duct means.

t t t; 5' t:

1. In apparatus for separating gas from particulate solid matterentrained therein and having a solids collection receptacle; a blower;means providing communication between the intake of said blower and saidsolids collection receptacle whereby to place the interior of saidsolids collection receptacle under relatively negative pressure; ahollow structure for receiving particulate solid matter for entrainmentin a gas flow stream; first duct means affording communication of theblower discharge with said hollow structure; and second duct meansaffording communication of said solids collection receptacle with saidhollow structure; said solids collection receptacle, said blower, saidhollow structure and said first and second duct means providing a systemfor recirculation of gas in a flow path from said hollow structurethrough said second duct means, said solids collection receptacle, saidblower and said fiRst duct means back to said hollow structure andcontinuously over said path, the intake of said blower, said solidscollection receptacle, said second duct means and a part of said hollowstructure being under said relatively negative pressure, whereby somegas in addition to the recirculated gas enters into said flow path fromoutside said system; the improvement comprising gas bleeding means forpreventing said addition of gas from outside said system fromundesirably increasing the average gas pressure in said system, said gasbleeding means including a pressure relief plenum; a bypass opening insaid first duct means communicating with said pressure relief plenum; agas bleed outlet in said plenum; and a valve in said plenum for variablycontrolling the flow of gas therethrough from said by-pass opening toand through said gas bleed outlet.
 2. Apparatus according to claim 1including a filter mounted to receive gas discharged through said gasbleed outlet.
 3. Apparatus according to claim 2 including a port in saidsecond duct means communicating with said plenum, said valve beingoperable for selectively placing said plenum respectively intocommunication with said by-pass opening in said first duct means andwith said port in said second duct means, communication of said plenumwith said port in said second duct means providing for reverse flow ofgas through said filter to dislodge solid material therefrom and carryit entrained through said second duct means to said solids collectionreceptacle.
 4. Apparatus according to claim 3 in which said filtercomprises a foraminous bag into which gas is discharged through said gasbleed outlet.
 5. Apparatus according to claim 1 in which said first ductmeans includes the casing of said blower, said by-pass opening beingformed in said casing itself.
 6. Apparatus according to claim 1 in whichsaid plenum includes an outer cylindrical wall portion located toreceive by-passed gas tangentially from said by-pass opening toestablish a swirling movement of gas and entrained particulate solidmatter in said plenum, said gas bleed outlet being at the centralportion of said plenum; said apparatus further including a port in saidsecond duct means communicating with said plenum at a zone spacedcircumferentially from the tangential reception of by-passed gas by saidplenum, said valve being selectively movable to two positions in one ofwhich positions it establishes relatively free flow of gas from saidby-pass opening into said plenum and substantially prevents free flow ofby-pass gas from said plenum to said port, and in the other of whichpositions said valve substantially prevents flow of gas from saidby-pass opening into said plenum and establishes relatively free flow ofgas from said plenum to said port and thence to said second duct meansand said solids collection receptacle.
 7. Apparatus according to claim 6including a foraminous bag filter mounted to receive gas discharged fromsaid gas bleed outlet.
 8. Apparatus according to claim 7 including meansproviding communication between said second duct means and the interiorof said bag filter at a location spaced from said gas bleed outlet. 9.Apparatus according to claim 6 in which said valve comprises a segmentalpart substantially conforming to the shape of said outer cylindricalwall portion, and means mounting said valve to turn about the axis ofsaid cylindrical wall portion and said segmental part.
 10. Apparatusaccording to claim 9 in which, when said valve is in said one of saidtwo positions, a portion of said segmental part mainly cuts offcommunication between said plenum and said port but an edge of saidsegmental portion is spaced slightly from said plenum wall to provide askimmer slit through which particulate solids centrifugally concentratedadjacent the inner surface of said plenum wall by the swirling of thegas within the plenum are discharged together with a minor portion ofthe by-passed gas to said port and thence to said Second duct means andsaid solids collection receptacle, the major portion of the by-passedgas being discharged through said gas bleed outlet.
 11. Apparatusaccording to claim 10 including a hollow filter for receiving by-passedgas discharged through said gas bleed outlet; and solids return meansfor conducting a portion only of the gas and solids delivered to saidfilter to said second duct means for return to said solids collectionreceptacle.
 12. Apparatus according to claim 11 in which said filterextends downwardly from said gas bleed outlet, and in which said solidsreutrn means is connected to the lower portion of said filter. 13.Apparatus according to claim 12 in which said solids return meanscommunicates with said second duct means via said hollow structure. 14.Apparatus according to claim 1 including a vertical screen partition insaid solids collection receptacle dividing the latter into two sectionsone of which communicates with the blower intake and the other of whichcommunicates with said second duct means.
 15. Apparatus according toclaim 1 including a separation chamber having a curved wall in opencommunication with said collection receptacle; a blower inlet passageextending through a wall of said chamber to the blower intake; and acurved baffle in said chamber having one edge spaced relatively far fromthe adjacent chamber wal to provide relatively free flow of gas fromsaid collection receptacle into said chamber for being swirled thereinas it moves inwardly to said inlet passage, the opposite edge of saidcurved baffle being spaced from but relatively close to said chambercurved wall to provide a skimmer slit through which solid mattercentrifugally concentrated in the outer portion of said chamber isreturned to said collection receptacle.
 16. Apparatus according to claim15 in which said blower comprises an impeller rotatable about ahorizontal axis, and in which said one edge of said curved baffle isspaced below the top of said separation chamber to provide therelatively free flow of gas from the upper portion of said collectionreceptacle into the upper portion of said separation chamber, andfurther in which said opposite edge of said curved baffle is at thelower portion of the separation chamber, whereby said skimmer slit is solocated as to discharge into the lower portion of said collectionreceptacle.
 17. In an air flow surface cleaning apparatus, a pick-uphead mounted for movement in the direction of cleaning progress over andadjacent to the surface to be cleaned, said pick-up head including apressure chamber; a suction chamber; an inclined partition between saidchambers extending transversely of the direction of cleaning progressmovement and having an inclined slit orifice for the discharge of highvelocity blast air from said pressure chamber against said surface andinto said suction chamber; and means at the ends of said slit orifice inthe path of the high velocity blast air for directing blast air emergingfrom said slit orifice inwardly and away from the respectively adjacentends of said suction chamber and toward the longitudinal center of saidsuction chamber.
 18. Apparatus according to claim 17 in which said meansat the ends of said slit orifice comprise spacers within said slitorifice.
 19. Apparatus according to claim 17 in which said means at theends of said slit orifice comprise members within said suction chamberat the opposite ends thereof and having horizontal webs with chamferedportions in the path of the blast air emerging from the ends of the slitorifice.
 20. Apparatus according to claim 17 in which said pick-up headhas a unitary top wall comprising a relatively high pressure chamber topwall part; a relatively low suction chamber top wall part; and anintervening step-down part.
 21. Apparatus according to claim 20 in whichsaid intervening step-down part is inclined at the same inclination assaid inclined partition.
 22. Apparatus according to claim 17 in wHichsaid pick-up head includes a horizontal wall forming the bottom of thepressure chamber and spaced above the level of the surface beingcleaned, said horizontal wall having an inclined edge flange providingone side of said slit orifice.
 23. In an air flow surface cleaningequipment, a mobile support; a pick-up head carried by said support andincluding a pressure chamber and a suction chamber; and apparatus forseparating air from particulate solid matter picked up and entrained byair flowing in said pick-up head from said pressure chamber to saidsuction chamber, said apparatus being mounted on said mobile support andcomprising a solids collection receptacle; a blower; means providingcommunication between the intake of said blower and said receptacle;first duct means affording communication of the blower discharge withsaid pick-up head pressure chamber; second duct means affordingcommunication of said receptacle with said pick-up head suction chamber,said receptacle, said blower, said pressure chamber, said suctionchamber and said first and second duct means providing a system forcontinuous recirculation of air in a flow path from said suction chamberthrough said second duct means, said receptacle, said blower and saidfirst duct means to said pressure chamber and thence back to saidsuction chamber, the intake of said blower, said solids collectionreceptacle, said second duct means and said pick-up head suction chamberbeing under relatively negative pressure, said recirculation flow beingaccompanied by some air entering into said flow path from outside saidsystem and being added to the recirculated air flowing in said flowpath; a pressure relief plenum; a by-pass opening in said first ductmeans communicating with said pressure relief plenum; an air bleedoutlet in said plenum; and a valve in said plenum for variablycontrolling the flow of air therethrough from said by-pass opening toand through said air bleed outlet.
 24. Apparatus according to claim 23including a filter mounted to receive air discharged thnough said airbleed outlet.
 25. Apparatus according to claim 24 including a port insaid second duct means communicating with said plenum, said valve beingoperable for selectively placing said plenum respectively intocommunication with said by-pass opening in said first duct means andwith said port in said second duct means, communication of said plenumwith said port in said second duct means providing for reverse flow ofair through said filter to dislodge solid material therefrom and carryit entrained through said second duct means to said solids collectionreceptacle.
 26. Apparatus according to claim 23 in which said first ductmeans includes the casing of said blower, said by-pass opening beingformed in said casing itself.
 27. Apparatus according to claim 23 inwhich said plenum includes an outer cylindrical wall portion located toreceive by-passed air tangentially from said by-pass opening toestablish a swirling movement of air and entrained particulate solidmatter in said plenum, said air bleed outlet being at the centralportion of said plenum; said apparatus further including a port in saidsecond duct means communicating with said plenum at a zone spacedcircumferentially from the tangential reception of by-passed air by saidplenum, said valve being selectively movable to two positions in one ofwhich positions it establishes relatively free flow of air from saidby-pass opening into said plenum and substantially prevents free flow ofby-pass air from said plenum to said port, and in the other of whichpositions said valve substantially prevents flow of air from saidby-pass opening into said plenum and establishes relatively free flow ofair from said plenum to said port and thence to said second duct meansand said solids collection receptacle.
 28. Apparatus according to claim27 in which said valve comprises a segmental part substantiallyconforming to the shape of said outer cylindrical wall portion, andmEans mounting said valve to turn about the axis of said cylindricalwall portion and said segmental part.
 29. Apparatus according to claim 8in which, when said valve is in said one of said two positions, aportion of said segmental part mainly cuts off communication betweensaid plenum and said port but an edge of said segmental portion isspaced slightly from said plenum wall to provide a skimmer slit throughwhich particulate solids centrifugally concentrated adjacent the innersurface of said plenum wall by the swirling of the air within the plenumare discharged together with a minor portion of the by-passed air tosaid port and thence to said second duct means and said solidscollection receptacle, the major portion of the by-passed air beingdischarged through said air bleed outlet.
 30. Apparatus according toclaim 29 including a hollow filter for receiving by-passed airdischarged through said air bleed outlet; and solids return means forconducting a portion only of the air and solids delivered to said filterto said second duct means for return to said solids collectionreceptacle.
 31. Apparatus according to claim 30 in which said filterextends downwardly from said air bleed outlet, and in which said solidsreturn means is connected to the lower portion of said filter. 32.Apparatus according to claim 31 in which said solids return meanscommunicates with said second duct means via said pick-up head suctionchamber.
 33. In apparatus for separating gas from particulate solidmatter entrained therein and having a solids collection receptacle; ablower; means providing communication between the intake of said blowerand said solids collection receptacle whereby to place the interior ofsaid solids collection receptacle under relatively negative pressure; ahollow structure for receiving particulate solid matter for entrainmentin a gas flow stream; first duct means affording communication of theblower discharge with said hollow structure; and second duct meansaffording communication of said solids collection receptacle with saidhollow structure; said solids collection receptacle, said blower, saidhollow structure and said first and second duct means providing a systemfor recirculation of gas in a flow path from said hollow structurethrough said second duct means, said solids collection receptacle, saidblower and said first duct means back to said hollow structure andcontinuously over said path, the intake of said blower, said solidscollection receptacle, said second duct means and a part of said hollowstructure being under said relatively negative pressure, whereby somegas in addition to the recirculated gas enters into said flow path fromoutside said system; the improvement comprising gas bleeding means forpreventing said addition of gas from outside said system fromundesirably increasing the average gas pressure in said system, said gasbleeding means including a by-pass opening in said first duct means; agas bleed outlet communicating with said by-pass opening; a filtermounted vertically to receive therein gas and any entrained particlesdischarged through said gas bleed outlet; and solids return means forconducting a portion of the gas and solids delivered to said filter fromthe bottom of said filter to said second duct means for return to saidsolids collection receptacle, said solids return means comprising arestricted gas passage for causing said portion of gas and solids toflow at a high velocity so as to entrain particles collecting at thebottom of said filter and conduct them to said second duct means.